Digital radiography imagers provide several advantages compared to traditional film-based x-ray imagers. For example, digital radiography imagers enable physicians to view images in real-time with display monitors, the ability to store and retrieve large amounts of digitized image data, and the ability to transfer image data over high-speed networks. However, problems exist with respect to capturing high-resolution images in a digital format.
One type of digital radiography imager is a flat panel imager that uses a scintillating material to convert x-rays to visible light. An x-ray is a relatively high-energy photon having wavelength in the approximate range from 0.05 angstroms to 100 angstroms. Visible light is electromagnetic radiation that has a wavelength in the range from about 3,900 to about 7,700 angstroms and that may be perceived by the unaided, human eye.
The flat panel imager is constructed as a panel with a matrix of photosensitive devices with readout electronics to transfer the light intensity of a pixel to a digital video signal for further processing or viewing. An x-ray scintillator placed on top of the photosensitive devices becomes sensitive to x-rays, and can be used in a variety of medical and industrial applications. FIG. 1A illustrates a cross-sectional view of a prior art flat panel imager. When a pattern of x-rays is applied to the top side of a scintillator, it produces visible light with diminishing intensity as the x-rays propagate deeper into the scintillator. Photosensitive devices capture the light produced in the scintillator, and the electrical signal is further amplified and processed.
The amount of applied x-rays converted to light depends, among other things, on the thickness of the scintillator. The thicker the scintillator, the greater the amount of light produced. However, as the scintillator gets thicker, less of the generated light reaches the photosensitive devices, because the brightest area is near the side of the scintillator that is opposite to the side that faces the photosensitive devices.
These scintillator properties contradict each other, and for every scintillator material, structure, and x-ray energy, there is an optimal thickness that produces maximum signal on the photosensitive devices. The thickness of the scintillator also affects the sharpness of the image produced, because the light generated in the scintillator diffuses in all directions and smoothes sharp edges in the x-ray pattern. As shown in FIG. 1A, the thicker scintillator diffuses light further away from the edge of the x-ray pattern, and subsequently degrades image quality.
The basic disadvantage of the prior art flat panel construction as illustrated in FIG. 1A is the low efficiency of transferring light, produced in the scintillator, to the photosensitive devices. In some cases, 60% or more of the light may not exit the scintillator. This reduces the electrical signal produced by the photosensitive devices, lowers the signal to noise ratio as well as spatial resolution in the images. Signal to noise ratio is the level of x-ray intensity detected by the imager relative to interferences caused by electrical noise or quantum x-ray noise. Spatial resolution is the ability to discern between small features of the image.
Various flat panel imager constructions have been attempted to improve the efficiency of the scintillator-based imagers. One flat panel configuration places a mirror on the x-ray side of the scintillator to return the light generated at the x-ray side of the scintillator back into the scintillator, and eventually to the photodiode devices. In another configuration, the scintillator material is grown to have columnar structure to reduce the horizontal (as illustrated in FIG. 1A) diffusion of the generated light, and thus improve image quality. However, in both configurations, the light diffuses within the scintillator, and even with a columnar scintillator, the image quality degrades. In some cases, to improve efficiency, the thickness of the scintillator may be increased to be several times bigger than the pixel size. However, the problem of light diffusion within the scintillator still exists.
Another type of a prior art digital radiography imager is a flat panel imager that uses semiconductor material to convert x-rays to electric charges directly, without an intermediate step of converting x-rays to visible light. FIG. 1B illustrates one example of a prior art flat panel imager that converts x-ray energy directly. The flat panel imager has semiconductor layer disposed between a top electrode layer and charge-collection electrode layer. An electric field is applied across semiconductor layer incident to the top electrode. As x-rays propagate through the semiconductor layer through the top electrode, it creates electric charges within the semiconductor layer that are drawn to the charge-collection layer. The charge is collected, amplified and quantified to a digital code for a corresponding pixel.
Analogous to the scintillator-based imager described in FIG. 1A, the thickness of the semiconductor layer may affect the charge collection efficiency of charge collection layer. Thus, depending on the semiconductor material used, it may be difficult and burdensome to achieve optimum semiconductor thickness.